When it is desired to display an image given in polar coordinates, such as a radar image, on a display device working in television mode, generally a device known as an image digital converter or IDC is used, having the essential role of converting the image in polar coordinates into an image in Cartesian coordinates, and of memorizing it in digital form, so a to provide for the matching between the rate at which the images are supplied and the rate at which the television image is refreshed.
In a standard way, an IDC comprises: a radar interface circuit which receives the video signals coming from the radar and provides for their digitalization; a coordinates conversion sub-set; a memory, called an image memory, wherein the radar information is stored in digital form in the format in which it will be displayed. Connected to the image memory, the IDC further comprises artificial persistence circuits which have the role of creating, for the pieces of information of the image memory for which there are no modifications due to the ageing, a persistence effect comparable to the one produced on a persistence tube where the brilliance of a point starts decreasing as soon as it is recorded. In a system such as this, the process of writing in the memory is the following one.
The video signal is formed by the responses (echos) to a pulse emitted by the radar. It is given at a frequency which is the frequency at which the emission of these pulses is repeated, called the pulse repetition frequency or PRF. It must be noted that the PRF is determined by the range of the radar. The video signals given at the PRF are hereinafter called "recurrences".
To do its conversion, the coordinates conversion sub-set receives "rotation signals" which have the role of indicating, at any instant, the angular position of the beam of the radar, which rotates regularly with respect to a reference direction, generally the North. The rotation signals are formed, firstly, by a North signal (N) which is a pulse, or blip, whenever the beam passes through the North and, secondly, by an angle increment blip (.epsilon.) indicating that the beam has rotated by 1/n.sup.th with respect to the preceding increment, if n increments correspond to 360.degree..
The angle increment blips .epsilon. are generally given by the antenna encoder, asynchronously with the recurrences. .theta. designates the resultant angle, at a given instant, made by the radar beam with the reference direction (North). The video information assigned to each of the n directions .theta. of the beam is hereinafter called a "radial".
When the radar system gives n angle increments, it is generally desired to display n radials. It is then necessary to assign the different recurrences to the different radials. The different points of each radial thus formed are then recorded in image memory at the addresses computed in Cartesian coordinates.
However, as the periodicity of the angle increments .epsilon. is given by the antenna, and as the PRF is determined, besides, by the range of the radar, it appears that, in certain cases, these values may be such that one of the following configurations exists:
either the frequency of the increments .epsilon. is too low as compared with the PRF: among the recurrences received at the PRF, there are then recurrences which correspond to no value whatsoever of .theta.. They are either not recorded in memory and are lost or they are grouped together beforehand in a way which may be not constant from one radial to another or from one antenna rotation to another, and recorded in memory. The result thereof is an error in the positioning of the echos. Furthermore, this error may vary from one antenna to another; PA0 or the frequency of the increments .epsilon. is too high as compared with the PRF and there are angle .theta. values for which there is no corresponding radar information (recurrences) available: there is then no memory recording for want of information. The drawback therein is, notably, an irregularity of the image for want of a radial from time to time, as well as the disturbance of the working of the remanence.
To avoid this latter drawback, we are led, in practice, to modify the PRF in a direction that leads to limiting the range of the radar.
Another approach is described in the European patent Application, published under n.degree. 0.192.562 on behalf of THOMSON-CSF, corresponding to U.S. Pat. No. 4,757,317, wherein the periodicity of the increments .epsilon. is converted into a signal .epsilon.' as a function of the PRF of the radar in order to prevent any loss of information. The conversion of coordinates and the assignment of the recurrences is then done according to the converted value .epsilon.'. This method amounts to restricting the number of effectively displayed radials as compared with the number of values of the angle .theta. given by the antenna, i.e. the number of radials that it may be possible to display, whence a restriction in the quality of the image displayed.